Volume 14, Issue 3 (May 1986)

Fatigue Crack Growth Threshold and Crack Opening of a Mild Steel

The effects of stress ratio, compressive peak stress, and cold rolling on the threshold stress intensity and crack closure level of a mild steel (SAE 1010) were investigated. As the stress ratio and the magnitude of the compressive peak stress were increased, the threshold level decreased linearly. The threshold stress intensity also decreased linearly as the yield strength was increased by cold rolling. Crack opening measurements showed that the measured threshold is composed of two parts: an intrinsic threshold stress intensity range, ΔKin, and an opening stress intensity, ΔKop. The opening stress intensity decreased with increasing magnitude of the compressive peak stress. The intrinsic threshold stress intensity range was not significantly affected by the compressive peak stress. Severe cold rolling decreased the opening stress in the near threshold region to near zero. In a constant amplitude load-controlled test, the ratio of the opening stress intensity to the maximum stress intensity decreased as the maximum stress intensity (crack length) increased, and no crack closure could be observed when the net stress approached the yield strength of the material.